Polyurethanes for osteoimplants

ABSTRACT

Biological-based polyurethanes and methods of making the same. The polyurethanes are formed by reacting a biodegradable polyisocyanate (such as lysine diisocyanate) with an optionally hydroxylated biomolecule to form polyurethane. The polymers formed may be combined with ceramic and/or bone particles to form a composite, which may be used as an osteoimplant.

This application claims benefit of U.S. Provisional Application No.60/444,759, filed Feb. 4, 2003, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

Vertebrate bone is a composite material composed of hydroxyapatite,collagen, and a variety of noncollagenous proteins, as well as embeddedand adherent cells. Vertebrate bone can be processed into an implantablebiomaterial, such as an allograft, for example, by removing the cells,leaving behind the mineral and extracellular matrix. The processed bonebiomaterial can have a variety of properties, depending upon thespecific processes and treatments applied to it, and may incorporatecharacteristics of other biomaterials with which it is combined. Forexample, bone-derived biomaterials may be processed into load-bearingmineralized grafts that support and integrate with the patient's bone ormay alternatively be processed into soft, moldable or flowabledemineralized bone biomaterials that have the ability to induce acellular healing response.

The use of bone grafts and bone substitute materials in orthopedicmedicine is well known. While bone wounds can regenerate without theformation of scar tissue, fractures and other orthopedic injuries take along time to heal, during which the bone is unable to supportphysiologic loading. Metal pins, screws, and meshes are frequentlyrequired to replace the mechanical functions of injured bone. However,metal is significantly stiffer than bone. Use of metal implants mayresult in decreased bone density around the implant site due to stressshielding. Furthermore, metal implants are permanent and unable toparticipate in physiological remodeling.

Following implantation, the host's own bone remodeling capabilitiespermit some bone grafts and certain bone substitute materials to remodelinto endogenous bone that in most cases is indistinguishable from thehost's own bone. In general, however, it is a limitation of allograftbone that larger allografts do not completely remodel, and residualallograft bone may persist at the graft site for many years orindefinitely, potentially acting as a stress riser and a possiblefracture site. The use of bone grafts is further limited by theavailability of tissue with the appropriate shape and size, as well asthe desired mechanical strength and degradation rate.

U.S. Pat. No. 6,294,187, the contents of which are incorporated hereinby reference, describes methods for preparing composites includingallogenic bone for use in load bearing orthopedic applications. It isdesirable to increase the strength of bone-reinforced composites byincreasing the strength of the matrix material while retaining theresorbable properties of the matrix. Furthermore, there is a need for anovel resorbable polymer capable of synergistically interacting withbone to make a true composite having mechanical characteristics of bothbone and polymer. There is also a need to develop resorbable polymersfor the production of bone/polymer composites where the polymer itselfhas osteopromotive or osteopermissive properties and contributes toosteointegration and remodeling of the composite. It is also desirableto develop implants that do not elicit undesirable immune responses fromthe recipient. There is also a need to provide composite grafts ofsuitable shape and size that maximize the utility of the graft tissue.

SUMMARY OF THE INVENTION

In one aspect, the invention is a biodegradable polyurethane composite.The composite comprises a polyurethane matrix and a reinforcementembedded in the matrix. The polyurethane matrix is formed by reaction ofa polyisocyanate (e.g., lysine diisocyanate, toluene diisocyanate,arginine diisocyanate, asparagine diisocyanate, glutamine diisocyanate,hexamethylene diisocyanate, hexane diisocyanate, methylene bis-p-phenyldiisocyanate, isocyanurate polyisocyanates, 1,4-butane diisocyanate,uretdione polyisocyanate, or aliphatic, alicyclic, or aromaticpolyisocyanates) with an optionally hydroxylated biomolecule (e.g., aphospholipids, fatty acid, cholesterol, polysaccharide, starch, or acombination or modified form of any of the above) to form abiodegradable polymer, while the reinforcement comprises bone or a bonesubstitute (e.g., calcium carbonate, calcium sulfate, calciumphosphosilicate, sodium phosphate, calcium aluminate, calcium phosphate,calcium carbonate, hydroxyapatite, demineralized bone, mineralized bone,or combinations or modified forms of any of these). The polyurethane maybe cross-linked. The polyisocyanate may be a diisocyanate. Thebiomolecule may be lecithin. The composite may comprise other materials,such as polycaprolactone, or a biomolecule, bioactive agent, or smallmolecule (e.g., lectins, growth factors, immunosuppresives, orchemoattractants). The reinforcement may be present in amounts of atleast 10, 30, 50, or 70 weight percent. The composite may have a wetcompressive strength in excess of that of the polyurethane alone, or mayhave a wet compressive strength of at least 3 MPa, 10 MPa, 50 MPa, 75MPa, or 100 MPa. The composite may be able to survive at least 10⁵fatigue cycles at 3 MPa when wet, or 10⁶ fatigue cycles at 25 MPa whenwet. The creep rate may be less than 15% in 24 hours at 3 MPa when wet,or less than 10% in 24 hours at 25 MPa when wet. The polyurethane maydegrade at a rate sufficient to permit generation of new tissue at an invivo implantation site. The degradation rate may be about 5%, 10%, or25% of the original composite weight per month in vivo. The maximumresolved strength in shear, compression, or tension may be at least 3MPa.

In another aspect, the invention is a biodegradable polyurethane. Thepolyurethane is formed by the reaction of a polyisocyanate (e.g., lysinediisocyanate, toluene diisocyanate, arginine diisocyanate, asparaginediisocyanate, glutamine diisocyanate, hexamethylene diisocyanate, hexanediisocyanate, methylene bis-p-phenyl diisocyanate, isocyanuratepolyisocyanates, 1,4-butane diisocyanate, uretdione polyisocyanate, oraliphatic, alicyclic, or aromatic polyisocyanates) with a mixture ofoptionally hydroxylated biomolecules. The mixture of optionallyhydroxylated biomolecules includes polysaccharides and lipids orphospholipids, and may include lecithin. The polyurethane may becross-linked. The polyisocyanate may be a diisocyanate. The polyurethanemay comprise other materials, such as polycaprolactone, or abiomolecule, bioactive agent, or small molecule (e.g., lectins, growthfactors, immunosuppresives, or chemoattractants). The polyurethane mayalso comprise a reinforcement (e.g., calcium carbonate, calcium sulfate,calcium phosphosilicate, sodium phosphate, calcium aluminate, calciumphosphate, calcium carbonate, hydroxyapatite, demineralized bone,mineralized bone, or combinations or modified forms of any of these),which may be at least 10, 30, 50, or 70 weight percent of the compositeso formed. The composite may have a wet compressive strength in excessof that of the polyurethane alone. The polyurethane may have a wetcompressive strength of at least 3 MPa, 10 MPa, 50 MPa, 75 MPa, or 100MPa. The polyurethane may be able to survive at least 10⁵ fatigue cyclesat 3 MPa when wet, or 10⁶ fatigue cycles at 25 MPa when wet. The creeprate may be less than 15% in 24 hours at 3 MPa when wet, or less than10% in 24 hours at 25 MPa when wet. The polyurethane may degrade at arate sufficient to permit generation of new tissue at an in vivoimplantation site. The degradation rate may be about 5%, 10%, or 25% ofthe original polyurethane weight per month in vivo. The maximum resolvedstrength in shear, compression, or tension may be at least 3 MPa.

In still another aspect, the invention is a nonresorbable, biocompatiblepolyurethane. The polyurethane is formed by reaction of a polyisocyanate(e.g., lysine diisocyanate, toluene diisocyanate, arginine diisocyanate,asparagine diisocyanate, glutamine diisocyanate, hexamethylenediisocyanate, hexane diisocyanate, methylene bis-p-phenyl diisocyanate,isocyanurate polyisocyanates, 1,4-butane diisocyanate, uretdionepolyisocyanate, or aliphatic, alicyclic, or aromatic polyisocyanates),with a polysaccharide biomolecule, and optionally also with a lipid orphospholipid. The polyurethane may be cross-linked. The polyisocyanatemay be a diisocyanate, and bay react with a hydroxyl group on thebiomolecule. The polyurethane may comprise other materials, such aspolycaprolactone, or a biomolecule, bioactive agent, or small molecule(e.g., lectins, growth factors, immunosuppresives, or chemoattractants).The polyurethane may also comprise a reinforcement (e.g., calciumcarbonate, calcium sulfate, calcium phosphosilicate, sodium phosphate,calcium aluminate, calcium phosphate, calcium carbonate, hydroxyapatite,demineralized bone, mineralized bone, or combinations or modified formsof any of these), which may be at least 10, 30, 50, or 70 weight percentof the composite so formed. The composite may have a wet compressivestrength in excess of that of the polyurethane alone. The polyurethanemay have a wet compressive strength of at least 3 MPa, 10 MPa, 50 MPa,75 MPa, or 100 MPa. The polyurethane may be able to survive at least 10⁵fatigue cycles at 3 MPa when wet, or 10⁶ fatigue cycles at 25 MPa whenwet. The creep rate may be less than 15% in 24 hours at 3 MPa when wet,or less than 10% in 24 hours at 25 MPa when wet. The polyurethane maydegrade at a rate sufficient to permit generation of new tissue at an invivo implantation site. The degradation rate may be about 5%, 10%, or25% of the original polyurethane weight per month in vivo. The maximumresolved strength in shear, compression, or tension may be at least 3MPa.

In yet another aspect, the invention is a method of making apolyurethane composite. The method comprises reacting a polyisocyanate(e.g., lysine diisocyanate, toluene diisocyanate, arginine diisocyanate,asparagine diisocyanate, glutamine diisocyanate, hexamethylenediisocyanate, hexane diisocyanate, methylene bis-p-phenyl diisocyanate,isocyanurate polyisocyanates, 1,4-butane diisocyanate, uretdionepolyisocyanate, or aliphatic, alicyclic, or aromatic polyisocyanates)with an optionally hydroxylated biomolecule (e.g., a phospholipids,fatty acid, cholesterol, polysaccharide, starch, or a combination ormodified form of any of the above) and a reinforcement comprising boneor a bone substitute (e.g., calcium carbonate, calcium sulfate, calciumphosphosilicate, sodium phosphate, calcium aluminate, calcium phosphate,calcium carbonate, hydroxyapatite, demineralized bone, mineralized bone,or combinations or modified forms of any of these), to form abiodegradable polymer matrix having particles of reinforcement embeddedtherein. Additional substances such as bioactive agents, biomolecules,or small molecules (e.g., lectins, growth factors, immunosuppresives, orchemoattractants) may also be added to the composite. Reacting mayinclude adding a chain extender or exposing the reactants to a catalyst(e.g., mild bases, strong bases, sodium hydroxide, sodium acetate,tin-containing compounds, or triethylenediamine 1,4-diaza(2,2,2)bicyclooctane), and may be carried out for a time period in the rangefrom about one minute to about four hours. It may also include reactingthe polyisocyanate and the biomolecule to form a prepolymer, mixing theprepolymer with the reinforcement to form a precomposite, and reactingthe precomposite to form a polyurethane composite (e.g., bycross-linking).

In a further aspect, the invention is a method of making a biodegradablepolyurethane, by reacting a polyisocyanate (e.g., lysine diisocyanate,toluene diisocyanate, arginine diisocyanate, asparagine diisocyanate,glutamine diisocyanate, hexamethylene diisocyanate, hexane diisocyanate,methylene bis-p-phenyl diisocyanate, isocyanurate polyisocyanates,1,4-butane diisocyanate, uretdione polyisocyanate, or aliphatic,alicyclic, or aromatic polyisocyanates) with a mixture of optionallyhydroxylated biomolecules, comprising polysaccharides and lipids orphospholipids. The method may further comprise adding a reinforcement(e.g., calcium carbonate, calcium sulfate, calcium phosphosilicate,sodium phosphate, calcium aluminate, calcium phosphate, calciumcarbonate, hydroxyapatite, demineralized bone, mineralized bone, orcombinations or modified forms of any of these) to the polyurethane toform a composite material, for example by reacting the polyisocyanateand the biomolecule to form a prepolymer, mixing the prepolymer with thereinforcement to form a precomposite, and reacting the precomposite(e.g., by cross-linking). Other substances, such as a bioactive agent,biomolecule, or small molecule (e.g., lectins, growth factors,immunosuppresives, or chemoattractants) may also be added to thepolymer. Reacting may include adding a chain extender or exposing thereactants to a catalyst (e.g., mild bases, strong bases, sodiumhydroxide, sodium acetate, tin-containing compounds, ortriethylenediamine 1,4-diaza(2,2,2) bicyclooctane), and may be carriedout for a time period in the range from about one minute to about fourhours.

In yet a further aspect, the invention is a method of making anonresorbable, biocompatible polyurethane polymer, by reacting apolyisocyanate (e.g., lysine diisocyanate, toluene diisocyanate,arginine diisocyanate, asparagine diisocyanate, glutamine diisocyanate,hexamethylene diisocyanate, hexane diisocyanate, methylene bis-p-phenyldiisocyanate, isocyanurate polyisocyanates, 1,4-butane diisocyanate,uretdione polyisocyanate, or aliphatic, alicyclic, or aromaticpolyisocyanates) with a biomolecule comprising a polysaccharide. Themethod may further comprise adding a reinforcement (e.g., calciumcarbonate, calcium sulfate, calcium phosphosilicate, sodium phosphate,calcium aluminate, calcium phosphate, calcium carbonate, hydroxyapatite,demineralized bone, mineralized bone, or combinations or modified formsof any of these) to the polyurethane to form a composite material, forexample by reacting the polyisocyanate and the biomolecule to form aprepolymer, mixing the prepolymer with the reinforcement to form aprecomposite, and reacting the precomposite (e.g., by cross-linking).Other substances, such as a bioactive agent, biomolecule, or smallmolecule (e.g., lectins, growth factors, immunosuppresives, orchemoattractants) may also be added to the polymer. Reacting may includeadding a chain extender or exposing the reactants to a catalyst (e.g.,mild bases, strong bases, sodium hydroxide, sodium acetate,tin-containing compounds, or triethylenediamine 1,4-diaza(2,2,2)bicyclooctane), and may be carried out for a time period in the rangefrom about one minute to about four hours.

Definitions

The term “biomolecules,” as used herein, refers to classes of molecules(e.g., proteins, amino acids, peptides, polynucleotides, nucleotides,carbohydrates, sugars, lipids, nucleoproteins, glycoproteins,lipoproteins, steroids, etc.) that are commonly found in cells andtissues, whether the molecules themselves are naturally-occurring orartificially created (e.g., by synthetic or recombinant methods). Forexample, biomolecules include, but are not limited to, enzymes,receptors, neurotransmitters, hormones, cytokines, cell responsemodifiers such as growth factors and chemotactic factors, antibodies,vaccines, haptens, toxins, interferons, ribozymes, anti-sense agents,plasmids, DNA, and RNA.

The term “biocompatible,” as used herein, is intended to describematerials that, upon administration in vivo, do not induce undesirablelong term effects.

As used herein, “biodegradable,” “bioerodable,” or “resorbable”materials are materials that degrade under physiological conditions toform a product that can be metabolized or excreted without damage toorgans. Biodegradable materials are not necessarily hydrolyticallydegradable and may require enzymatic action to fully degrade.Biodegradable materials also include materials that are broken downwithin cells.

“Polynucleotide,” “nucleic acid,” or “oligonucleotide”: The terms“polynucleotide,” “nucleic acid,” or “oligonucleotide” refer to apolymer of nucleotides. The terms “polynucleotide”, “nucleic acid”, and“oligonucleotide”, may be used interchangeably. Typically, apolynucleotide comprises at least three nucleotides. DNAs and RNAs arepolynucleotides. The polymer may include natural nucleosides (i.e.,adenosine, thymidine, guanosine, cytidine, uridine, deoxyadenosine,deoxythymidine, deoxyguanosine, and deoxycytidine), nucleoside analogs(e.g., 2-aminoadenosine, 2-thiothymidine, inosine, pyrrolo-pyrimidine,3-methyl adenosine, C5-propynylcytidine, C5-propynyluridine,C5-bromouridine, C5-fluorouridine, C5-iodouridine, C5-methylcytidine,7-deazaadenosine, 7-deazaguanosine, 8-oxoadenosine, 8-oxoguanosine,O(6)-methylguanine, and 2-thiocytidine), chemically modified bases,biologically modified bases (e.g., methylated bases), intercalatedbases, modified sugars (e.g., 2′-fluororibose, ribose, 2′-deoxyribose,arabinose, and hexose), or modified phosphate groups (e.g.,phosphorothioates and 5′-N-phosphoramidite linkages).

“Polypeptide”, “peptide”, or “protein”: According to the presentinvention, a “polypeptide,” “peptide,” or “protein” comprises a stringof at least two amino acids linked together by peptide bonds. The terms“polypeptide”, “peptide”, and “protein”, may be used interchangeably.Peptide may refer to an individual peptide or a collection of peptides.Inventive peptides preferably contain only natural amino acids, althoughnon-natural amino acids (i.e., compounds that do not occur in nature butthat can be incorporated into a polypeptide chain; see, for example,http://www.cco.caltech.edu/˜dadgrp/Unnatstruct.gif, which displaysstructures of non-natural amino acids that have been successfullyincorporated into functional ion channels) and/or amino acid analogs asare known in the art may alternatively be employed. Also, one or more ofthe amino acids in an inventive peptide may be modified, for example, bythe addition of a chemical entity such as a carbohydrate group, aphosphate group, a farnesyl group, an isofarnesyl group, a fatty acidgroup, a linker for conjugation, functionalization, or othermodification, etc. In a preferred embodiment, the modifications of thepeptide lead to a more stable peptide (e.g., greater half-life in vivo).These modifications may include cyclization of the peptide, theincorporation of D-amino acids, etc. None of the modifications shouldsubstantially interfere with the desired biological activity of thepeptide.

The terms “polysaccharide,” “carbohydrate,” “oligosaccharide,” or“starch” refer to a polymer of sugars. The terms “polysaccharide” and“carbohydrate” may be used interchangeably to mean a sugar polymer ofany length. “Oligosaccharide” generally refers to a relatively lowmolecular weight polymer, while “starch” typically refers to a highermolecular weight polymer. The polymer may include natural sugars (e.g.,glucose, fructose, galactose, mannose, arabinose, ribose, and xylose)and/or modified sugars (e.g., 2′-fluororibose, 2′-deoxyribose, andhexose). Polysaccharides may or may not be crosslinked.

“Small molecule”: As used herein, the term “small molecule” is used torefer to molecules, whether naturally-occurring or artificially created(e.g., via chemical synthesis), that have a relatively low molecularweight. Typically, small molecules have a molecular weight of less thanabout 5000 g/mol. Preferred small molecules are biologically active inthat they produce a local or systemic effect in animals, preferablymammals, more preferably humans. In certain preferred embodiments, thesmall molecule is a drug. Preferably, though not necessarily, the drugis one that has already been deemed safe and effective for use by theappropriate governmental agency or body. For example, drugs for humanuse listed by the FDA under 21 C.F.R. §§330.5, 331 through 361, and 440through 460; drugs for veterinary use listed by the FDA under 21 C.F.R.§§500 through 589, incorporated herein by reference, are all consideredacceptable for use in accordance with the present invention.

As used herein, “bioactive agents” is used to refer to compounds orentities that alter, inhibit, activate, or otherwise affect biologicalor chemical events. For example, bioactive agents may include, but arenot limited to, anti-AIDS substances, anti-cancer substances,antibiotics, immunosuppressants (e.g., cyclosporine), anti-viralsubstances, enzyme inhibitors, neurotoxins, opioids, hypnotics,anti-histamines, lubricants, tranquilizers, anti-convulsants, musclerelaxants and anti-Parkinson substances, anti-spasmodics and musclecontractants including channel blockers, miotics and anti-cholinergics,anti-glaucoma compounds, anti-parasite and/or anti-protozoal compounds,modulators of cell-extracellular matrix interactions including cellgrowth inhibitors and anti-adhesion molecules, vasodilating agents,inhibitors of DNA, RNA or protein synthesis, anti-hypertensives,analgesics, anti-pyretics, steroidal and non-steroidal anti-inflammatoryagents, anti-angiogenic factors, anti-secretory factors, anticoagulantsand/or antithrombotic agents, local anesthetics, ophthalmics,prostaglandins, anti-depressants, anti-psychotic substances,anti-emetics, imaging agents, specific targeting agents,neurotransmitters, proteins, cell response modifiers, and vaccines. In acertain preferred embodiments, the bioactive agent is a drug.

A more complete listing of bioactive agents and specific drugs suitablefor use in the present invention may be found in “PharmaceuticalSubstances: Syntheses, Patents, Applications” by Axel Kleemann andJurgen Engel, Thieme Medical Publishing, 1999; the “Merck Index: AnEncyclopedia of Chemicals, Drugs, and Biologicals”, Edited by SusanBudavari et al., CRC Press, 1996, the United StatesPharmacopeia-25/National Formulary-20, published by the United StatesPharmcopeial Convention, Inc., Rockville Md., 2001, and the“Pharmazeutische Wirkstoffe,” edited by Von Keemann et al.,Stuttgart/New York, 1987, all of which are incorporated herein byreference.

As used herein, “anti-AIDS substances” are substances used to treat orprevent Autoimmune Deficiency Syndrome (AIDS). Examples of suchsubstances include CD4,3′-azido-3′-deoxythymidine (AZT),9-(2-hydroxyethoxymethyl)-guanine acyclovir, phosphonoformic acid,1-adamantanamine, peptide T, and 2′,3′ dideoxycytidine.

As used herein, “anti-cancer substances” are substances used to treat orprevent cancer. Examples of such substances include methotrexate,cisplatin, prednisone, hydroxyprogesterone, medroxyprogesterone acetate,megestrol acetate, diethylstilbestrol, testosterone propionate,fluoxymesterone, vinblastine, vincristine, vindesine, daunorubicin,doxorubicin, hydroxyurea, procarbazine, aminoglutethimide,mechlorethamine, cyclophosphamide, melphalan, uracil mustard,chlorambucil, busulfan, carmustine, lomusline, dacarbazine (DTIC:dimethyltriazenomidazolecarboxamide), methotrexate, fluorouracil,5-fluorouracil, cytarabine, cytosine arabinoxide, mercaptopurine,6-mercaptopurine, and thioguanine.

As used herein, “antibiotics” are substances which inhibit the growth ofor kill microorganisms. Antibiotics can be produced synthetically or bymicroorganisms. Examples of antibiotics include penicillin,tetracycline, chloramphenicol, minocycline, doxycycline, vanomycin,bacitracin, kanamycin, neomycin, gentamycin, erythromicin andcephalosporins.

As used herein, “anti-viral agents” are substances capable of destroyingor suppressing the replication of viruses. Examples of anti-viral agentsinclude a-methyl-P-adamantanemethylamine,1,-D-ribofuranosyl-1,2,4-triazole-3 carboxamide,9-[2-hydroxy-ethoxy] methylguanine,adamantanamine, 5-iodo-2′-deoxyuridine, trifluorothymidine, interferon,and adenine arabinoside.

As used herein, “enzyme inhibitors” are substances which inhibit anenzymatic reaction. Examples of enzyme inhibitors include edrophoniumchloride, N-methylphysostigmine, neostigmine bromide, physostigminesulfate,tacrine HCl, tacrine, 1-hydroxy maleate,iodotubercidin,p-bromotetramisole,10-(alpha-diethylaminopropionyl)-phenothiazine hydrochloride,calmidazolium chloride, hemicholinium-3,3,5-dinitrocatechol,diacylglycerol kinase inhibitor I, diacylglycerol kinase inhibitor II,3-phenylpropargylamine, N6-monomethyl-L-arginine acetate, carbidopa,3-hydroxybenzylhydrazine HCl, hydralazine HCl, clorgyline HCl,deprenylHCl, L(−)-, deprenylHCl, D(+)-, hydroxylamine HCl, iproniazidphosphate, 6-MeO-tetrahydro-9H-pyrido-indole, nialamide, pargylineHCl,quinacrineHCl, semicarbazide HCl, tranylcypromine HCl,N,N-diethylaminoethyl-2,2-diphenylvalerate hydrochloride,3-isobutyl-1-methylxanthne, papaverine HCl, indomethacind,2-cyclooctyl-2-hydroxyethylamine hydrochloride,2,3-dichloro-a-methylbenzylamine (DCMB),8,9-dichloro-2,3,4,5-tetrahydro-1H-2-benzazepine hydrochloride,p-aminoglutethimide, p-aminoglutethimide tartrate,R(+)-,p-aminoglutethimide tartrate, S(−)-,3-iodotyrosine,alpha-methyltyrosine, L-,alpha-methyltyrosine, D L-,acetazolamide,dichlorphenamide, 6-hydroxy-2-benzothiazolesulfonamide, and allopurinol.

As used herein, “neurotoxins” are substances that have a toxic effect onthe nervous system, e.g. on nerve cells. Neurotoxins include adrenergicneurotoxins, cholinergic neurotoxins, dopaminergic neurotoxins, andother neurotoxins. Examples of adrenergic neurotoxins includeN-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride. Examples ofcholinergic neurotoxins include acetylethylcholine mustardhydrochloride. Examples of dopaminergic neurotoxins include6-hydroxydopamine HBr, 1-methyl-4-(2-methylphenyl)-1,2,3,6-tetrahydro-pyridine hydrochloride,1-methyl-4-phenyl-2,3-dihydropyridinium perchlorate,N-methyl-4-phenyl-1, 2,5,6tetrahydropyridine HCl,1-methyl-4-phenylpyridinium iodide.

As used herein, “opioids” are substances having opiate-like effects thatare not derived from opium. Opioids include opioid agonists and opioidantagonists. Opioid agonists include codeine sulfate, fentanyl citrate,hydrocodone bitartrate, loperamide HCl, morphine sulfate, noscapine,norcodeine, normorphine, thebaine. Opioid antagonists includenor-binaltorphimine HCl, buprenorphine,chlornaltrexamine2HCl,funaltrexamione HCl, nalbuphine HCl,nalorphine HCl, naloxone HCl,naloxonazine, naltrexone HCl, andnaltrindole HCl.

As used herein, “hypnotics” are substances, which produce a hypnoticeffect. Hypnotics include pentobarbital sodium, phenobarbital,secobarbital, thiopental and mixtures thereof, heterocyclichypnotics,dioxopiperidines, glutarimides, diethyl isovaleramide, a-bromoisovalerylurea, urethanes and disulfanes.

As used herein, “antihistamines” are substances which competitivelyinhibit the effects of histamines. Examples include pyrilamine,chlorpheniramine, tetrahydrazoline, and the like.

As used herein, “lubricants” are substances that increase the lubricityof the environment into which they are delivered. Examples ofbiologically active lubricants include water and saline.

As used herein, “tranquilizers” are substances which provide atranquilizing effect. Examples of tranquilizers include chloropromazine,promazine, fluphenzaine, reserpine, deserpidine, and meprobamate.

As used herein, “anti-convulsants” are substances which have an effectof preventing, reducing, or eliminating convulsions. Examples of suchagents include primidone, phenytoin, valproate, Chk and ethosuximide.

As used herein, “muscle relaxants” and “anti-Parkinson agents” areagents which relax muscles or reduce or eliminate symptoms associatedwith Parkinson's disease. Examples of such agents include mephenesin,methocarbomal, cyclobenzaprine hydrochloride, trihexylphenidylhydrochloride, levodopa/carbidopa, and biperiden.

As used herein, “anti-spasmodics” and “muscle contractants” aresubstances capable of preventing or relieving muscle spasms orcontractions. Examples of such agents include atropine, scopolamine,oxyphenonium, and papaverine.

As used herein, “miotics” and “anti-cholinergics” are compounds whichcause bronchodilation. Examples include echothiophate, pilocarpine,physostigmine salicylate, diisopropylfluorophosphate, epinephrine,neostigmine, carbachol, methacholine, bethanechol, and the like.

As used herein, “anti-glaucoma compounds” are compounds for theprevention or treatment of glaucoma, and include betaxalol, pilocarpine,timolol, timolol salts, and combinations of timolol, and/or its salts,with pilocarpine.

As used herein, “anti-parasitics”, “anti-protozoals”, and “anti-fungals”are compounds for the prevention or treatment of infestations ofparasites, protozoa, and fungi, and include ivermectin, pyrimethamine,trisulfapyrimidine,clindamycin, amphotericin B, nystatin, flucytosine,natamycin, and miconazole.

As used herein, “anti-hypertensives” are substances capable ofcounteracting high blood pressure. Examples of such substances includealpha-methyldopa and the pivaloyloxyethyl ester of alpha-methyldopa.

As used herein, “analgesics” are substances capable of preventing,reducing, or relieving pain. Examples of analgesics include morphinesulfate, codeine sulfate, meperidine, and nalorphine.

As used herein, “anti-pyretics” are substances capable of relieving orreducing fever, and “anti-inflammatory agents” are substances capable ofcounteracting or suppressing inflammation. Examples of such substancesinclude aspirin (salicylic acid), indomethacin, sodiumindomethacintrihydrate, salicylamide, naproxen, colchicines, fenoprofen,sulindac, diflunisal, diclofenac, indoprofen and sodium salicylamide.

As used herein, “local anesthetics” are substances which have ananesthetic effect in a localized region. Examples of such anestheticsinclude procaine, lidocaine, tetracaine and dibucaine.

As used herein, “ophthalmics” include diagnostic agents such as sodiumfluorescein, rose bengal, methacholine, adrenaline, cocaine, andatropine. Ophthalmic surgical additives include alpha-chymotrypsin andhyaluronidase.

As used herein, “prostaglandins” are an art-recognized class ofnaturally occurring chemically related, long-chain hydroxy fatty acidsthat have a variety of biological effects.

As used herein, “anti-depressants” are substances capable of preventingor relieving depression. Examples of anti-depressants includeimipramine, amitriptyline, nortriptyline, protriptyline, desipramine,amoxapine, doxepin, maprotiline, tranylcypromine, phenelzine, andisocarboxazide.

As used herein, “anti-psychotic substances” are substances which modifypsychotic behavior. Examples of such agents include phenothiazines,butyrophenones and thioxanthenes.

As used herein, “anti-emetics” are substances which prevent or alleviatenausea or vomiting. An example of such a substance is dramamine.

As used herein, “imaging agents” are agents capable of imaging a desiredsite, e.g., tumor, in vivo. Examples of imaging agents includesubstances having a label which is detectable in vivo, e.g., antibodiesattached to fluorescent labels. The term antibody includes wholeantibodies or fragments thereof.

As used herein, “specific targeting agents” include agents capable ofdelivering a therapeutic agent to a desired site, e.g., a tumor, andproviding a therapeutic effect. Examples of targeting agents includeagents which can carry toxins or other agents which provide beneficialeffects. The targeting agent can be an antibody linked to a toxin, e.g.,ricin A, or an antibody linked to a drug.

As used herein, “neurotransmitters” are substances which are releasedfrom a neuron on excitation and travel to either inhibit or excite atarget cell. Examples of neurotransmitters include dopamine, serotonin,q-aminobutyric acid, norepinephrine, histamine, acetylcholine, andepinephrine.

As used herein, “cell response modifiers” are chemotactic factors suchas platelet-derived growth factor (PDGF). Other chemotactic factorsinclude neutrophil-activating protein, monocyte chemoattractant protein,macrophage-inflammatory protein, platelet factor, platelet basicprotein, and melanoma growth stimulating activity; epidermal growthfactor, transforming growth factor (alpha), fibroblast growth factor,platelet-derived endothelial cell growth factor, insulin-like growthfactor, nerve growth factor, and bone growth/cartilage-inducing factor(alpha and beta), or other bone morphogenetic protein. Other cellresponse modifiers are the interleukins, interleukin inhibitors orinterleukin receptors, including interleukin 1 through interleukin 10;interferons, including alpha, beta and gamma; hematopoietic factors,including erythropoietin, granulocyte colony stimulating factor,macrophage colony stimulating factor and granulocyte-macrophage colonystimulating factor; tumor necrosis factors, including alpha and beta;transforming growth factors (beta), including beta-1, beta-2, beta-3,inhibin, and activin; and bone morphogenic proteins including all BMPs.

The term “shaped,” as applied to the osteoimplant herein, refers to adetermined or regular form or configuration, in contrast to anindeterminate or vague form or configuration (as in the case of a lumpor other solid mass of no special form) and is characteristic of suchmaterials as sheet, plate, particle, sphere, hemisphere strand, coiledstrand, capillary network, film, fiber, mesh, disk, cone, portion of acone, pin, screw, tube, cup, tooth, tooth root, strut, wedge, portion ofwedge, cylinder, threaded cylinder, rod, hinge, rivet, anchor, spheroid,ellipsoid, oblate spheroid, prolate ellipsoid, hyperbolic paraboloid,and the like.

The phrase “wet compressive strength,” as utilized herein, refers to thecompressive strength of the osteoimplant after the osteoimplant has beenimmersed in physiological saline (water containing 0.9 g NaCl/100 mlwater) for a minimum of 12 hours. Compressive strength is a well-knownmeasurement of mechanical strength and is measured using the proceduredescribed herein.

The terms “osteogenic,” or “osteopromotive,” as applied to theosteoimplant of this invention, shall be understood as referring to theability of the osteoimplant to enhance or accelerate the ingrowth of newbone tissue by one or more mechanisms such as osteogenesis,osteoconduction and/or osteoinduction.

The term “osteopermissive,” as applied to the osteoimplant of thisinvention, shall be understood as referring to the ability of theosteoimplant to not impede the ingrowth of new bone tissue by one ormore mechanisms such as osteogenesis, osteoconduction and/orosteoinduction.

As utilized herein, the phrase “superficially demineralized” as appliedto bone particles refers to bone particles possessing at least about 90weight percent of their original inorganic mineral content. The phrase“partially demineralized” as applied to the bone particles refers tobone particles possessing from about 8 to about 90 weight percent oftheir original inorganic mineral content, and the phrase “fullydemineralized” as applied to the bone particles refers to bone particlespossessing less than about 8, for example, less than about 1, weightpercent of their original inorganic mineral content. The unmodified term“demineralized” as applied to the bone particles is intended to coverany one or combination of the foregoing types of demineralized boneparticles.

Unless otherwise specified, all material proportions used herein are inweight percent.

The term “polyisocyanate,” as that term is used herein, encompasses anychemical structure comprising two or more cyanate groups. A“diisocyanate,” as used herein, is a subset of the class ofpolyisocyanates, a chemical structure containing exactly two cyanate(—CN) groups. Similarly, a “polyol” contains two or more alcohol (—OH)groups, while a “diol” contains exactly two alcohol groups.

The term “polyurethane,” as used herein, is intended to include allpolymers incorporating more than one urethane group (—NH—CO—O—) in thepolymer backbone. Polyurethanes are commonly formed by the reaction of apolyisocyanate (such as a diisocyanate) with a polyol (such as a diol):

Polyurethanes may be straight chains or branched, and may have high orlow molecular weights. The R₁ and R₂ groups provide great flexibility intailoring the mechanical and chemical properties of polyurethanes, whichmay be made rigid, soft, plastic, and/or elastomeric by selection ofappropriate functional groups.

As used herein, the term “composite” refers to a mixture of two or moredifferent materials, denominated “matrix” and “reinforcement.” Multiplereinforcement materials may be present in a single composite. The term“reinforcement” is not intended to limit or describe any mechanicalproperties of a material so denominated or its contribution to themechanical properties of the composite. While the material denominatedas the “matrix” may act as a binder to hold together particles, fibers,or fragments of reinforcement material(s), it is not required that thematrix material be fully interconnected throughout the composite;neither is it assumed that the reinforcement material is or is notinterconnected throughout the composite. The terms “matrix” and“reinforcement” are also not limited by the fraction of each materialpresent in the composite.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

According to the present invention, polyurethane materials are formed byadding an appropriate polyisocyanate crosslinker (e.g., a diisocyanate)to biomolecules such as lipids (e.g., phospholipids, lecithin, fattyacids, or cholesterols, any of which may be hydroxylated to improvepolymerization) polysaccharides (e.g., oligosaccharides oramylase-resistant starches), and/or bone. These polyurethane materialsmay be mixed with calcium carbonate, calcium sulfate, calciumphosphosilicate, sodium phosphate, calcium aluminate, calcium phosphate,calcium carbonate, hydroxyapatite, other ceramics, or bone, to formcomposites, which preferably have osteopromotive, osteogenic, and/orosteoinductive properties. Details of traditional polyurethane synthesiscan be found, for example, in Lamba, et al., Polyurethanes in biomedicalapplications, CRC Press, 1998, which is incorporated herein byreference, and particularly in chapter 2 of the above reference.

It is preferred that the polyurethane component of the compositereaction be resorbable and biocompatible. Zhang et al. have synthesizeda lysine diisocyanate ethyl ester which they have found to bebiocompatible (see Zhang, et al., “A new peptide-based urethane polymer:synthesis, biodegradation, and potential to support cell growth invitro,” Biomaterials 21: 1247-1258 (2000), and Zhang, et al.,“Synthesis, Biodegradability, and Biocompatibility of LysineDiisocyanate-Glucose Polymers,” Tis. Eng., 8(5): 771-785 (2002), both ofwhich are incorporated herein by reference). Polyurethanes made fromthis diisocyanate or any other polyisocyanate (e.g., toluenediisocyanate, arginine diisocyanate, asparagine diisocyanate, prolinediisocyanate, glutamine diisocyanate, hexamethylene diisocyanate, hexanediisocyanate, methylene bis-p-phenyl diisocyanate, isocyanuratepolyisocyanates, 1,4-butane diisocyanate, uretdione polyisocyanate,aliphatic, alicyclic, or aromatic polyisocyanates) that are degradableby the host and does not have undesirable toxic effects in vivo may beused to prepare the polyurethanes and composites of the invention.

The polyol component of the polyurethane of the invention is abiomolecule, which may be hydroxylated by standard methods if it doesnot already possess sufficient hydroxyl groups to carry out a reaction.For example, lipids, including phospholipids, mono-, di-, andtriglycerides, fatty acids, and cholesterols, may require addition ofhydroxyl groups in order to carry out the polymeriztion reaction. Incontrast, polysaccharides such as starches typically already havesufficient hydroxyl groups to polymerize readily into a highlycross-linked polymer. The biomolecule polyol may be mixed with otherpolyols. For example, poly(ε-caprolactone) is a common additive whensynthesizing polyurethane block copolymers, and may also be used in thepresent invention. Other polycaprolactones may also be eithercopolymerized or blended into the final polymer, as may otherappropriate polymers.

When a diol is reacted with a diisocyanate, a polyurethane with minimalcrosslinking is generally formed. Such polymers are generallythermoplastic and readily deformable, and may be subject tostrain-induced crystallization for hardening. In contrast, if at leastsome of either the polyol or the polyisocyanate comprises at least threeactive groups participating in the reaction, then the polymer willgenerally be heavily cross-linked. Such polymers are typicallythermosets, and tend to be harder than polymers with low cross-linking.In addition, their mechanical properties tend to be less dependent onhow they are processed, which may render them more machinable.

Because the reaction process combines an isocyanate with a biologicalmolecule, any breakdown products of the polymer are generallybiocompatible and preferably resorbable. It is preferred that thepolyurethanes of the invention be enzymatically degradable, bioerodable,hydrolytically stable, and/or bioabsorbable. Thus, when an osteoimplantis formed from the materials of the invention, it can be slowly replacedby the ingrowth of natural bone as the implant degrades. This process ofosteogenesis may be accelerated, for example, by the addition ofbioactive agents. Such bioactive agents may be incorporated into thepolymer structure, either as backbone elements or as side groups, orthey may be present as solutes in the solid polymer or as non-covalentlybonded attachments. In any case, they may be gradually released as thepolyurethane degrades. The rate of release may be tailored by modifyingthe attachment or incorporation of the bioactive agents into thepolymer. Bioactive agents that may be used include not only agentshaving osteogenic properties, but also agents having other biologicalproperties such as immunosuppression, chemoattraction, or those listedin Appendix A. Lectins are a class of particular interest forincorporation into the present polymers, especially when the polymerscomprise carbohydrates, which bond readily to lectins.

In some embodiments, it is preferred that the polyurethanes of theinvention be enzymatically degradable, bioerodable, hydrolyticallystable, and/or bioabsorable. Thus, when an osteoimplant formed from thematerials of the invention degrades, any byproducts of the enzymaticprocess or bioerosion may be biocompatible and may be utilized in or maybe metabolites in any cellular metabolic pathway, such as but notlimited to cellular respiration, glycolysis, fermentation, or thetricarboxylic acid cycle.

For certain applications, it may be desirable to create foamedpolyurethane, rather than solid polyurethane. While typical foamingagents such as hydrochlorofluoro-carbons, hydrofluorocarbons, andpentanes may not be biocompatible for many systems, other, biocompatibleagents may be used. For example, Zhang et al. have found that water maybe an adequate foaming agent for a lysine diisocyanate/PEG/glycerolpolyurethane (see Zhang, et al., “Three-dimensional biocompatibleascorbic acid-containing scaffold for bone tissue engineering,” supra).Other foaming agents include dry ice or other agents which releasecarbon dioxide or other gases into the composite.

Whether foamed or solid, polyurethanes according to the invention may beformed into a composite with bone particulates (optionallydemineralized), or with bone substitutes such as calcium carbonate,calcium sulfate, calcium phosphosilicate, sodium phosphate, calciumaluminate, calcium phosphate, calcium carbonate, hydroxyapatite, orother ceramics. In addition, collagen may also be formed into acomposite with the polyurethane, with or without the addition of bone.The treatment of bone particles for incorporation into composites isdiscussed below. It is noted that natural bone, hydroxyapatite, andcollagen may bond strongly to the isocyanates used in forming thepolymer, since they contain significant numbers of active hydroxylgroups. Thus, it may be preferred in some embodiments to first mix thebone, hydroxyapatite, and/or collagen with the polyol monomer, beforeaddition of the isocyanate. Nevertheless, it is also within the scope ofthe invention to mix the reinforcing material into already-combinedpolyol and isocyanate, or to combine all three componentssimultaneously.

The polyurethanes and composites of the invention preferably have asufficient wet compressive strength to provide mechanical stability foran osteoimplant during healing. In addition, they preferably have lowcreep rates and good fatigue resistance. For example, wet compressivestrengths of at least 3 MPa, 10 MPa, or 50 MPa are preferred, withstrengths of at least 75 MPa or 100 MPa being even more desirable. Creeprates of less than 10% per 24 hours at 25 MPa (wet) are preferred, as isfatigue resistance of at least 10⁶ cycles at 25 MPa (wet). However, evenif these mechanical properties are not present in the polymer orcomposite, the polymers and composites of the invention can be combinedwith other materials or used alone in osteoimplants according to theinvention. In some preferred embodiments, the mechanical strengeth,elastic modulus, and anisotropic properties of the implant can betailored by adjusting the polymer chain length distribution, side chainlength, degree of cross-linking, and/or physical processing.

Preparation of Bone for Incorporation into Composites

The bone particles employed in the preparation of the boneparticle-containing composition can be obtained from cortical,cancellous, and/or corticocancellous bone which may be of autogenous,allogenic and/or xenogeneic origin and may or may not contain cellsand/or cellular components. Preferably, the bone particles are obtainedfrom cortical bone of allogenic origin. Porcine and bovine bone areparticularly advantageous types of xenogeneic bone tissue that can beused individually or in combination as sources for the bone particles.Particles are formed by milling whole bone to produce fibers, chippingwhole bone, cutting whole bone, fracturing whole bone in liquidnitrogen, or otherwise disintegrating the bone tissue. Particles canoptionally be sieved to produce particles of a specific size.

The bone particles employed in the composition can be virtually anyfragment or portion of a whole bone, such as powdered bone particlespossessing a wide range of particle sizes ranging from relatively finepowders to coarse grains and even larger chips, cubes, shards, orfibers. In one embodiment, bone particles can range in average particlesize from about 0.05 mm to about 1.2 mm and possess a median length tomedian thickness ratio of from about less than 1:1 to about greater than10:1. In another embodiment, bone particles can range in averageparticle size from about 0.005 mm to about 10 mm and possess a medianlength to median thickness ration from about less than 1:1 to aboutgreater than 100:1. If desired, powdered bone particles can be gradedinto different sizes to reduce or eliminate any less desirable size(s)of particles that may be present. The combination of bone particles anda polymer both reduces the amount of bone that is required to preparethe implant and eliminates shape constraints on the bone itself, sincethe polymer and composite may be molded into any desired shape.

Alternatively, or in combination with the aforementioned bone powder,bone particles generally characterized as elongate and possessingrelatively high median length to median thickness ratios can be utilizedherein. Such elongate particles can be readily obtained by any one ofseveral methods, e.g., by milling or shaving the surface of an entirebone or relatively large section of bone. Employing a milling technique,one can obtain a mass of elongate bone particles containing, forexample, at least about 60 weight percent of elongate bone particlespossessing a median length of from about 2 to about 200 mm or more, amedian thickness of from about 0.05 to about 2 mm, and a median width offrom about 1 mm to about 20 mm. Such elongate bone particles can possessa median length to median thickness ratio of at least about 50:1 up toabout 500:1 or more and a median length to median width ratio of fromabout 10:1 to about 200:1. The milling process may be optimized toadjust the size of the bone particles and the size distribution, andvirtually any fragment or portion of a whole bone could be made by themilling process. The mechanical strength, elastic modulus, andanisotropy of the implant can be tailored by adjusting the weightpercent of the various shapes (elongate, particulate, etc.) of boneparticles utilized in the composite.

Another procedure for obtaining elongate bone particles, particularlyuseful for pieces of bone of up to about 100 mm in length, is the boneprocessing mill described in commonly assigned U.S. Pat. No. 5,607,269,the entire contents of which are incorporated herein by reference. Useof this bone mill results in the production of long, thin strips thatquickly curl lengthwise to provide tubular-like bone particles. Ifdesired, elongate bone particles can be graded into different sizes toreduce or eliminate any less desirable size(s) of particles that may bepresent. In overall appearance, elongate bone particles can be describedas filaments, fibers, threads, slender or narrow strips, etc.

The bone particles are optionally demineralized in accordance with knownand conventional procedures in order to reduce their inorganic mineralcontent. Demineralization methods remove the inorganic mineral componentof bone, for example by employing acid solutions. Such methods are wellknown in the art, see for example, Reddi, et al., Proc. Nat. Acad. Sci.,1972, 69:1601-1605, the contents of which are incorporated herein byreference. The strength of the acid solution, the shape of the boneparticles and the duration of the demineralization treatment willdetermine the extent of demineralization. Reference in this regard maybe made to Lewandrowski, et al., J. Biomed. Mater. Res., 1996, 31:365-372, the contents of which are also incorporated herein byreference.

In a preferred demineralization procedure, the bone particles aresubjected to a defatting/disinfecting step, followed by an aciddemineralization step. A preferred defatting/disinfectant solution is anaqueous solution of ethanol. Ethanol is a good solvent for lipids, andwater is a good hydrophilic carrier that enables the solution topenetrate more deeply into the bone particles. Ordinarily, at leastabout 10 to about 40 percent by weight of water (i.e., about 60 to about90 weight percent of defatting agent such as alcohol) should be presentin the defatting/disinfecting solution to produce optimal lipid removaland disinfection within the shortest period of time. The preferredconcentration range of the defatting solution is from about 60 to about85 weight percent alcohol and most preferably about 70 weight percentalcohol. Following defatting, the bone particles are immersed in acidover time to effect their demineralization. The acid also disinfects thebone by killing viruses, vegetative microorganisms, and spores. Acidsthat can be employed in this step include inorganic acids such ashydrochloric acid and organic acids such as peracetic acid. After acidtreatment, the demineralized bone particles are rinsed with sterilewater to remove residual amounts of acid and thereby raise the pH. Thebone particles are preferably dried, for example, by lyophilization,before incorporated into the composite. The bone particles may be storedunder aseptic conditions until they are used or sterilized using knownmethods shortly before combining them with the monomer.

Mixtures or combinations of one or more of the above types of boneparticles can be employed. For example, one or more of the foregoingtypes of demineralized bone particles can be employed in combinationwith nondemineralized bone particles, i.e., bone particles that have notbeen subjected to a demineralization process. The demineralized boneparticles may behave as short fibers in the composite, acting toincrease fracture toughness. The nondemineralized bone particles maybehave as ceramic inclusions, increasing the compressive strength of thecomposite. Nondemineralized bone is itself a fiber-reinforced composite,which may increase the bending and tensile stress the composite canwithstand before the bone particles break. Superficial demineralizationproduces particles containing a mineralized core. Particles of this typemay behave as non-demineralized particles in the composite, depending onthe degree on demineralization.

Bone particles may either be used without lyophilization or lyophilizedand/or otherwise treated to remove water from the bone. Some preferredembodiments of the described invention include the use of lyophilizedbone.

The bone particles in the composite also play a biological role.Non-demineralized bone particles bring about new bone ingrowth byosteoconduction, in which an advancing bone front binds to the particlesurface. Demineralized bone particles likewise play a biological role inbringing about new bone ingrowth by osteoinduction, in which bone cellsare recruited from the host tissue to regenerate bone at the implantsite. Both types of bone particles may be gradually remodeled andreplaced by new host bone as degradation of the composite progressesover time. This process is desirable because the load-bearing capacityis gradually transferred from the implant to the new bone growth,thereby reducing the risk of implant failure due to rapid degradation.

EXAMPLES Example #1

To determine the compressive strength of a composite implant made ofapproximately 66.6% bone and 33.3% castor bean polyurethane resin; 20grams of bovine bone powder (particle size 120 μm-500 μm) were combinedwith a two part polyurethane (Doctors Research Group, Plymouth Conn. anddescribed in “Vegetal Polyurethane Resin Implant Cranioplasty.Experimental Studies in Rabbits” by Luiz Fernando Francisco, Sao Jose doRio Preto, 1998, which is incorporated herein by reference). Firstly,6.10 grams of liquid comprising a polyisocyanate terminated molecule“prepolymer” were combined with 3.60 grams of a liquid comprising castorbean oil fatty acid triglyceride “diol”. Next, bone particles weregradually mixed into the polyurethane solution, until the bone appearedwell coated. The mixture was then packed by hand into three 5 ccsyringes (packed with light hand pressure). The samples were then setaside to polymerize over a 48-hour period at room temperature. Afterpolymerization was complete, the samples were removed from the syringesand cut to length (approx. 16 mm). Of the 4 samples tested; 2 weretested dry, while two were hydrated in Simulated Body Fluid (SBF) for 24hours and tested wet. (SBF solution contained 7.992-7.998 NaCl,0.2230-0.2243 g KCl, 0.2275-0.2289 g K₂HPO₄.3H₂O, 0.3041-0.3059 gMgCl₂.6H₂O, 36-40 ml HCl (1N), 0.3665-0.3687 g CaCl₂.2H₂O, 0.0708-0.0712g Na₂SO₄, 0.3517-0.3539 g NaHCO₃, and deionized water to make 1000 ml,adjusted to a pH of 7.2-7.4 by a buffer solution oftris(hydroxymethyl)aminomethane). The results of mechanical staticcompression tests using the Bionix MTS 858 (Edin Prarrie Minn.) areshown in column 5 of Table 1. Results indicated a slight decrease incompressive strength (of about 7%) with the hydrated implants comparedto the compressive strength of the dry implants, but load bearingcapacity was still considered acceptable for use as an implant.

TABLE 1 Compressive Sample Length (mm) Diameter (mm) Weight (g) Strength(MPa) A-Dry 16.74 11.85 2.70 72 B-Dry 16.58 11.84 2.64 72 C-Wet 16.6811.87 2.63 66 D-Wet 16.70 11.87 2.63 67

Example #2

To determine the compressive strength of an implant made of 100%two-part castor bean polyurethane resin, (Doctors Research Group,Plymouth Conn. and described in “Vegetal Polyurethane Resin ImplantCranioplasty. Experimental Studies in Rabbits” by Luiz FernandoFrancisco, Sao Jose do Rio Preto, 1998) enough of the prepolymer anddiol (as indicated in Example 1) were mixed together to fill a 5 ccsyringe. The material was hand packed into the syringe and allowed topolymerize for 18 hours at room temperature (air bubbles were noticedwithin the sample). After polymerization was complete, the samples wereremoved from the syringe and cut to length (approx. 13 mm). The resultsof mechanical static compression tests, using the Bionix MTS 858 (EdinPrarrie Minn.), are shown in column 5 of Table 2. The MPa values listedare only approximate at the point of visible plastic deformation of theimplant. Samples did not mechanically fail at 20 MPa, but ratherplastically deformed such that the test had to be stopped atapproximately 50% strain. The load bearing capacity of the implants wasstill considered acceptable for use as an implant.

TABLE 2 Approximate Compressive Sample ID Length (mm) Diameter (mm)Weight (g) Strength (MPa) A-Dry 12.96 8.55 .78 20 B-Dry 13.97 8.52 .8120

Example #3

To determine the compressive strength of a composite implant made ofapproximately 75% bone and 25% castor bean polyurethane resin, 20 gramsof bovine bone powder (particle size 120 μm-500 μm) were combined with a6.82 grams of a two part polyurethane (Doctors Research Group, PlymouthConn. and described in “Vegetal Polyurethane Resin Implant Cranioplasty.Experimental Studies in Rabbits” by Luiz Fernando Francisco, Sao Jose doRio Preto, 1998). The mixture was then packed by hand into three 5 ccsyringes (packed with light hand pressure). The samples were then setaside to polymerize over a 48-hour period at room temperature. Afterpolymerization was complete, the samples were removed from the syringesand cut to length (approx. 14 mm). Of the 6 samples tested; 4 weretested dry, while two were hydrated in Simulated Body Fluid (SBF) for 24hours and tested wet. The results of mechanical static compression testsusing the Bionix MTS 858 (Edin Prarrie Minn.) are shown in column 5 ofTable 3. Results indicated a decrease in compressive strength (of about21.8%) with the hydrated implants compared to the compressive strengthof the dry implants but load bearing capacity was still consideredacceptable for use as an implant.

TABLE 3 Compressive Sample ID Length (mm) Diameter (mm) Weight (g)Strength (MPa) A1-Dry 13.92 11.88 2.03 51 A2-Dry 14.02 11.87 2.14 56A3-Wet 12.37 11.96 1.96 43 B1-Dry 14.16 11.86 2.25 59 B2-Dry 14.16 11.812.11 54 B3-Wet 14.34 11.92 2.23 43

Example #4

To determine if a polyurethane could be made using a lecithin and acastor bean polyurethane resin, 3.0 grams of lecithin powder werecombined with a 3.0 grams of liquid comprising a polyisocyanateterminated molecule “prepolymer” (as indicated in Example 1). Themixture was then packed by hand into 5 cc syringes (packed with lighthand pressure). While the sample did polymerize, the reaction took morethan 48 hours.

Example #5

To determine if composite implant compressive strength could beincreased by improving the association and/or number of urethane bondsof the bone particles and the “diol”, an implant comprising 73% boneparticles and 23% two-part castor bean polyurethane resin, (as inExample 1) was made by first mixing 15 grams of demineralized bonepowder (particle size 120 μm-500 μm) with the “diol” as indicated inExample 1. The mixture was allowed to sit for 1 hour to ensure that “thediol” penetrated into the bone. Next, the liquid comprising apolyisocyanate terminated molecule “prepolymer” was mixed into thematerial and hand packed into 5 cc syringes. After polymerization wascomplete the material was removed from the syringe, but fell apart. Thismay have been due to excess diol or lack of sufficient prepolymer.Modifications of this method will result in an implant that maintainsits shape and is suitable for implantation.

Example #6

To determine if a polyurethane could be made using a Toluenediisocyanate and a castor bean polyurethane resin, 4.0 grams of aToluene diisocyanate were combined with 4.0 grams of a liquid comprisingcastor bean oil fatty acid triglyceride “diol” as indicated inExample 1. The mixture was then packed by hand into 5 cc syringes(packed with light hand pressure). While the sample did partiallypolymerize, the material was not firm. Addition of a catalyst mayincrease the rate of and efficiency of polymerization in this example.This example was also performed with 65% Toluene diisocyanate and 35%diol, again the sample did at least partially polymerize. The reactiontook more than 48 hours, but the material was not firm.

Example #7

To determine if a polyurethane could be made using a Toluenediisocyanate and a hydroxylated lecithin, 4.0 grams of a Toluenediisocyanate were combined with 4.0 grams of a hydroxylated lecithin.The mixture was then packed by hand into 5 cc syringes (packed withlight hand pressure). The sample did at least partially polymerizefaster than in Example 6, but the material was not firm.

Example #8

To determine if a polyurethane could be made using a Toluenediisocyanate and a hydroxylated lecithin with the addition of heat toimprove the rate of the polymerization, a 50:50 mixture was produced asin Example 7, while being heated to 93-95 degrees Celsius (on hotplate). The material became foamy and flowed over the mixing vessel.Once the material cooled it formed a porous at least partiallypolymerized sheet.

Example #9

To determine if a polyurethane could be made using a lysine diisocyanateand a hydroxylated lecithin, 6.0 grams of a lysine diisocyanate werecombined with 6.0 grams of a hydroxylated lecithin. The mixture was thenset at room temperature to polymerize. While the sample did at leastpartially polymerize with a hard shell after 72 hours, the material wasnot firm.

Example #10

To determine if a polyurethane could be made using a lysine diisocyanateand a hydroxylated lecithin, 12.0 grams of a lysine diisocyanate werecombined with 4.0 grams of a hydroxylated lecithin. The mixture was thenset at room temperature to polymerize. While the sample did polymerizevery quickly, it swelled up, filled with air bubbles generating foamthat developed a hard shell after a few hours.

Example #11

To determine if a composite implant could be made of bone with a lysinediisocyanate and castor bean polyurethane resin; 6 grams of a lysinediisocyanate were combined with 3.50 grams of a liquid comprising castorbean oil fatty acid triglyceride “the diol”. Next, the mixture washeated to 93-95 degrees Celsius (on hot plate) and bone particles(particle size 120 μm-500 μm) were gradually mixed into the polyurethanesolution, until the bone appeared well coated. The mixture was thenpacked by hand into 5 cc syringes (packed with light hand pressure). Thesamples were then set aside to polymerize over a 48-hour period at roomtemperature. The material polymerized at least partially and could beextruded out of the syringe.

Example #12

3 grams of lysine diisocyanate were mixed with ProGenix Carrier #2 andat least partially polymerized to produce a flexible gel like sheetwithin a few hours.

Example #13

3 grams of lysine diisocyanate were mixed with 1.5 grams glycerol. After2 weeks the mixture formed a hard at least partially polymerized filmlayer.

Example #14

6 grams of lysine diisocyanate were combined with 3 grams of starchcarrier B90 and M180 (Grain Processing Corporation, Muscatine, Iowa).When mixture was partially polymerized, 1.5 grams of bone (particle size120 μm-500 μm) were added to create a slurry. The material was then handpacked into a 5 cc syringe and pressed lightly with plunger. Althoughthe materially may have at least partially polymerized, it remained softand flexible.

Example #15

To demonstrate polymerization according to the invention, a monomer ormonomer combination, is mixed with bone. Desired formulations by weightpercent are given in Table 4. Ratios of crosslinker to polymer may bevaried according to specific requirements of the desired biomaterialover a wide range, at least from about 10:1 to 1:10. A conventionalpolymerization catalyst known to those skilled in the art (such as anamine or tin compound) may or may not also be added, and the mixture isthen combined with the crosslinking agent indicated and placed in a mold(such as Teflon) to polymerize. The percentage of the final compositecomprised of composite filler (i.e., bone) may be varied between 5% and95% according to the specific requirements of the biomaterial. Themixture polymerizes to form a bone-polyurethane composite. In onepreferred embodiment calcium phosphate granules are substituted for thebone portion of the formulation. Exemplary preparations of calciumphosphates are described by U.S. Pat. No. 5,650,176 to Lee et al., U.S.Pat. No. 6,002,065 to Constantz et al., and U.S. Pat. No. 6,206,957 toDriessens et al., all of which are incorporated by reference herein.

TABLE 4 Formulation Monomer Crosslinker number (wt %) (wt %)Reinforcement 1 Lecithin Hexamethylene Cortical bone 2 StarchDiisocyanate particles (200- 3 Starch:Lecithin 15:85 1000 microns) 4Starch:Lecithin 85:15 5 Collagen 6 Lecithin Uretdione 7 Starchpolyisocyanate 8 Starch:Lecithin 15:85 9 Starch:Lecithin 85:15 10Collagen 11 Lecithin 1,4 butane 12 Starch diisocyanate 13Starch:Lecithin 15:85 14 Starch:Lecithin 85:15 15 Collagen 16 LecithinHexamethylene Surface 17 Starch diisocyanate demineralized 18Starch:Lecithin 15:85 bone particles 19 Starch:Lecithin 85:15 20Collagen 21 Lecithin Uretdione 22 Starch polyisocyanate 23Starch:Lecithin 15:85 24 Starch:Lecithin 85:15 25 Collagen 26 Lecithin1,4 butane 27 Starch diisocyanate 28 Starch:Lecithin 15:85 29Starch:Lecithin 85:15 30 Collagen 31 Lecithin Hexamethylene Calcium 32Starch diisocyanate Phosphate 33 Starch:Lecithin 15:85 34Starch:Lecithin 85:15 35 Collagen 36 Lecithin Uretdione 37 Starchpolyisocyanate 38 Starch:Lecithin 15:85 39 Starch:Lecithin 85:15 36Collagen 37 Lecithin 1,4 butane 38 Starch diisocyanate 39Starch:Lecithin 15:85 40 Starch:Lecithin 85:15 41 Collagen

Other embodiments of the invention will be apparent to those skilled inthe art from a consideration of the specification or practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with the true scope and spiritof the invention being indicated by the following claims.

What is claimed is:
 1. A biodegradable polyurethane composite,comprising: a polyurethane matrix formed by reaction of a polyisocyanatewith a hydroxylated biomolecule to form a biodegradable polyurethanepolymer; and a reinforcement embedded in the matrix, wherein thereinforcement comprises a material selected from the group consisting ofbone and bone substitutes.
 2. The polyurethane composite of claim 1,wherein the reinforcement comprises a material selected from the groupconsisting of calcium carbonate, calcium sulfate, calciumphosphosilicate, sodium phosphate, calcium aluminate, calcium phosphate,calcium carbonate, hydroxyapatite, demineralized bone, mineralized bone,and combinations and modified forms of the above.
 3. The polyurethanecomposite of claim 1, wherein the biodegradable polyurethane polymer iscross-linked.
 4. The polyurethane composite of claim 1, wherein thepolyisocyanate is a diisocyanate.
 5. The polyurethane composite of claim1, wherein the polyisocyanate is selected from the group consisting oflysine diisocyanate, toluene diisocyanate, arginine diisocyanate,asparagine diisocyanate, glutamine diisocyanate, hexamethylenediisocyanate, hexane diisocyanate, methylene bis-p-phenyl diisocyanate,isocyanurate polyisocyanates, 1,4-butane diisocyanate, uretdionepolyisocyanate, and aliphatic, alicyclic, and aromatic polyisocyanates.6. The polyurethane composite of claim 1, wherein the biomolecule isselected from the group consisting of phospholipids, fatty acids,cholesterols, polysaccharides, starches, and combinations and modifiedforms of the above.
 7. The polyurethane composite of claim 1, whereinthe biomolecule is lecithin.
 8. The polyurethane composite of claim 1,further comprising polycaprolactone.
 9. The polyurethane composite ofclaim 1, further comprising one or more substances selected from abiomolecule, a bioactive agent, and a small molecule.
 10. Thepolyurethane composite of claim 9, wherein the substance is selectedfrom the group consisting of lectins, growth factors,immunosuppressives, and chemoatttractants.
 11. The polyurethanecomposite of claim 1, comprising at least 10 weight percent of thereinforcement.
 12. The polyurethane composite of claim 1, comprising atleast 30 weight percent of the reinforcement.
 13. The polyurethanecomposite of claim 1, comprising at least 50 weight percent of thereinforcement.
 14. The polyurethane composite of claim 1, comprising atleast 70 weight percent of the reinforcement.
 15. The polyurethanecomposite of claim 1, wherein the polyurethane composite has a wetcompressive strength that exceeds the wet compressive strength of thepolyurethane alone.
 16. The polyurethane composite of claim 1, whereinthe polyurethane composite has a wet compressive strength of at least 3MPa.
 17. The polyurethane composite of claim 1, wherein the polyurethanecomposite has a wet compressive strength of at least 10 MPa.
 18. Thepolyurethane composite of claim 1, wherein the polyurethane compositehas a wet compressive strength of at least 50 MPa.
 19. The polyurethanecomposite of claim 1, wherein the polyurethane composite has a wetcompressive strength of at least 75 MPa.
 20. The polyurethane compositeof claim 1, wherein the polyurethane composite has a wet compressivestrength of at least 100 MPa. 21.-111. (canceled)